1. Field of the Invention
The present invention relates to an optical lens system for taking image, and more particularly to a miniaturized optical lens system for taking image used in a digital camera.
2. Description of the Prior Art
In recent years, with the popularity of the mobile phone camera, the optical lens system for taking image has become thinner and thinner, and the electronic imaging sensor of a general digital camera is typically a CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) sensor. Due to advances in semiconductor manufacturing, the pixel size of sensors has been reduced continuously, and miniaturized optical lens systems for taking image have increasingly higher resolution. Therefore, there's increasing demand for image quality.
A conventional mobile phone camera described usually consists of three lens elements: from the object side to the image side: a first lens element with positive refractive power, a second lens element with negative refractive power and a third lens element with positive refractive power, such as the optical lens system for taking image described in U.S. Pat. No. 7,262,965.
However, the third lens element of the above patent is positive in such a manner that the principal point is close to the image side of the system, so the back focal length is relatively long, which results in a relatively large mechanical structure. As a result, the requirement of miniaturization can not be satisfied.
An optical lens system for taking image which uses lens elements made of glass and plastic is described in U.S. Pat. No. 6,490,102, wherein the third lens element is a spherical glass lens element. The use of the spherical lens element reduces the degrees of freedom available for correcting the off-axis aberration of the optical lens system, making it more difficult to control image quality.
The present invention mitigates and/or obviates the afore-mentioned disadvantages.